How far can intelligence agencies go, when collecting data, before it is deemed a breach of privacy?

In light of recent events, I feel this is a topic worthy of proper intellectual debate.

Intelligence agencies have a duty to collect information for policymakers and to present the threats,risks,hazards that are present to our society and state.

The increased globalisation, use of social media and connectivity means that intelligence agencies have developed their SIGINT (Signals Intelligence) capabilities to enable them to focus on the WWW. This has proved to be vital in combating terrorism/subversive activities that may affect the state security.